Intensive work has been done in the last two decades on the development of high performance lithium ion batteries, especially on polymer based lithium ion batteries (W. H. Meyer, Advanced Materials, V10. No. 6, 439 (1998); M. Z. A. Munish, in Handbook of Solid State Batteries and Capacitors, Chapter 19. Ed. By M. Z. A. Munish, World Scientific Pub. Singapore, 1995). These include two major strategies on using ion-conducting polymers as electrolyte in lithium batteries have been used. The first strategy is the development of highly conductive materials via the cross linking of mobile chains to form networks, which are then swollen by lithium salt solutions or electrolyte. The matrix in which the ion transport occurs is liquid-like. One example of gel electrolyte is the polymer battery (PLiON™) developed by Gozdz et al (A. S. Gozdz, C. N. Schmutz, and J.-M. Tarascon, U.S. Pat. No. 5,296,318, Mar. 22, 1994; A. S. Gozdz, C. N. Schmutz, J. M. Tarascon, and P. C. Warren, U.S. Pat. No. 5,456,000, Oct. 10, 1995; J. M. Tarascon, A. S. Gozdz, C. Schmutz, F. K. Shokoohi, P. C. Warren, Solid State Ionics, 86, 49 (1996); A. S. Gozdz, T. Falls, C. N. Schmutz, and P. C. Warren, U.S. Pat. No. 5,587,253, Dec. 24, 1996). In this case, the liquid electrolyte is absorbed in a polymer membrane based on polyvinylidene fluoride (PVDF) polymer in a manner similar to a sponge holding water. Although these batteries can be prepared in prismatic form, their market access is still hindered by the safety concerns and degassing problems associated with liquid electrolyte used in the batteries.
The second strategy in the development of polymer batteries is the construction of solid polymer electrolytes (SPEs) with reasonable conductivity but without the addition of a liquid electrolyte. One polymer widely investigated is poly (ethylene oxide) or PEO, which is able to form stable complexes with a number of salts (D. E. Fenton, J. M. Parker, P. V. Wright, Polymers, 14,589 (1973). It exhibits low ionic conductivity ranging from 10−9 to 10−8 S/cm at ambient temperature. Steady improvements over PEO based polymer electrolytes have been made since Fenton et al. early work in 1973. For example, Munishi and Zafer (M. Munishi and a. Zafar, international publication number WO 01/17051. Mar. 8, 2001; M. Munishi and A. Zafar, international publication number WO 01/17052. Mar. 8, 2001) reported polymer electrolytes that have a room temperature conductivity of between 10−5 to 10−4 S/cm. It consisted of a base polymer material with the plasticizer salt, inorganic filler and glassy or ceramic lithium ion conductor. Although these new polymers may improve the room temperature properties of lithium ion batteries, their operating temperature range (10 to 70° C.) is still very limited. For example, a PEO based electrolyte has a crystalline temperature range of 65-70° C. These polymers show a rapid decline in conductivity below the crystalline temperature. In fact, a PEO—LiClO4 electrolyte exhibits an ionic conductivity at ambient temperature of about 10−9 to 10−4 S/cm. Therefore, the batteries using these polymer electrolytes have to operate at more than 100° C. in order to be of any use. Another disadvantage of these polymer-based batteries is that they cannot use lithium metal as the anode although lithium has the highest energy density among all anode materials. This is because these polymers do not have enough mechanical strength to prevent lithium dendrite growth during the charge/discharge process.
Significant progress has been achieved in the last decade on the development of inorganic, solid state electrolytes. One of the best examples is the glassy lithium phosphorus oxynitride (“LiPON”) electrolyte developed by Bates et al in Oak Ridge National Laboratory (John Bates, Nancy Dudney, Greg Gruzalski, and Christopher Luck, U.S. Pat. No. 5,338,625, Aug. 16, 1994; X. Yu, J. B. Bates, G. E. Jellison, and B. C. Sales, J. Electrochem. Soc. 144, 524 (1997); B. J. Neudecker, R. A. Zuhr, in Intercalation Compounds for Battery Materials. Ed. By G. A. Nazri, M. Thackery, and T. Ohzuku, Electrochemical Society Proceeding V. 99-24, page 295). This solid-state electrolyte has a typical composition of Li2.9PO3.3N0.36 and is deposited by sputtering from a hot pressed Li3PO4 target in a nitrogen environment. It has a conductivity of 2*10−6 S/cm and is stable in contact with metallic lithium at potentials from 0 to nearly 5.5 V. Thin film batteries that used this electrolyte have demonstrated a cycle life of more than 40,000 full depth of charge-discharge cycles which is unthinkable for any other type of batteries. Thin film battery designs are also more flexible in their use of materials. Since a thin layer of material has a lower electrical resistance, it is possible to use some materials that could not be used in conventional bulk batteries (C. Julien and G. A. Nazri, “Solid State Batteries: Materials Design and Optimization,” Boston: Kluwer, 1994, p579).
To date, magnetron sputtering is the primary method used in the thin film battery industry for LiPON deposition. Magnetron sputtering is a well-established manufacturing method for depositing metal films at high rates, typically greater than 600 Å/min. However, the deposition rate from sputtering ceramic targets typically is low, at approximately 100 Å/min for LiPON film. Therefore, the cost of producing LiPON films is still prohibitively high for large-scale applications. Several other deposition techniques have been investigated for the preparation of LiPON films. For example, Vereda et al (Fernado Verda, Ronald B. Goldner, Terry E. Hass, and Peter Zerigian, Electrochemical and Solid-State Letters, 5(11) A239 (2002)) and Jenson et al (Mark Lynn Jenson and Victor Henry Weiss, U.S. patent application Ser. No. 815,983/09 Mar. 23, 2001) used an Ion Beam Assisted Deposition method to prepare LiPON films, but this method is difficult to scale up due to high equipment cost. Attempts to deposit electrolyte films by reactive electron beam evaporation in N2 environment were not successful. These results indicate that the formation of Lipon not only relies on the stoichiometry, but also on the special microstructure determined by the preparation conditions.
In summary, polymer electrolyte films still cannot satisfy the requirement for many advanced applications due to their limited operating temperature range and instability. Although solid electrolyte (LiPON) developed by Bates et al can overcome this instability problem, the high cost of the sputtering process has hindered its large-scale applications. Therefore, there is an urgent need for a solid-state electrolyte that exhibits a high ionic conductivity, wide operating temperature range, and can be prepared by a low cost production method. It is to the provision of such need that the present invention is primarily directed.